The present invention relates to a driver for data transmission, and particularly to a driver which provides frequency dependent pre-emphasis. It further relates to a data transmission system employing the driver, and a method for calibrating the system.
The frequency response of a interconnect (e.g. a PCB/cable) used for data transmission attenuates the high frequency components of a data stream. The level of this attenuation is illustrated schematically in FIG. 1, showing that it arises at above a frequency labelled xcfx892. When the attenuation starts below the maximum frequency of the data stream, this leads to a phenomenon known as inter-symbol interference (ISI), in which certain data patterns cause thinning of the data pulse widths, and hence there is data dependent jitter. A worst case example of this is for a data pattern in which the transmitted data has a single data high amongst data lows, as shown in FIG. 2(a). The attenuation of the high frequency components of the single data high results in a pulse which has reduced amplitude and long rise/fall times. Consequently pulse widths are reduced as shown in FIG. 2(b).
The CML type driver, illustrated in FIG. 3, includes a differential pair at the transmitter and loads at the receiver. The two inputs (which are equal and opposite voltage signals) are amplified by an amplifier 1, and then fed as equal and opposite signals txn and txp to respective transistors 3,5. The transmission lines 7, 9 have an impedance Z0. The loads R are at the termination for the transmission lines, connecting them to a voltage Vdd. An rx amp 10 is provided at the output. The gain of the driver is proportional to the width of the transistors 3,5 in the differential pair. Wider transistors provide more gain. A CML-type driver illustrated in FIG. 3 provides equal gain at all frequencies.
However, drivers are known which provide a pre-emphasis which flattens the frequency response of the interconnect at higher frequencies, preferably up to and beyond the data signal frequency. This is illustrated in FIG. 4, in which FIG. 4(a) is the frequency response of the interconnect, FIG. 4(b) is the frequency response of the pre-emphasis filter which has a pole at xcfx892, and FIG. 4(c) is the frequency response of the combination of the two, which is flat up to frequency xcfx891. The pre-emphasis filter has a zero at xcfx892 to cancel the attenuation of the interconnect. The pre-emphasis filter also has a pole at xcfx891 but this is higher than the maximum data frequency. Such known drivers employ passive components to achieve the frequency response shown in FIG. 4(b). This means that, taking the pre-emphasis and data transmission period as a whole, low and high frequency components are both attenuated by the same amount, and little or no thinning of the pulse widths occurs.
The present invention seeks to provide a new and useful driver for data transmission.
In general terms, the present invention proposes a system in which data encoded in two equal and opposite input signals is transmitted along two transmission lines (which each extend from a first location to a second location) as a difference between currents on the two lines. Each input signal is used to regulate (modulate) a current which passes along a different respective one of the lines, and, additionally, relatively low frequency components of each input signal component are used to regulate a current which is directed along the other respective one of the lines. Thus, the difference in the total currents on the two lines is greater for high frequency components than for low-frequency components.
Specifically, the first expression of the present is a data transmission system including:
a pair of electrical transmission lines;
a current source connected to each of said lines;
a transmission regulator which is arranged to receive data as two opposite input signals, and to transmit the data along the transmission lines as a difference between the currents at a first end of the lines; and
measurement means for measuring the difference between the currents at a second end of the lines, and from the difference extracting the data;
wherein the transmission regulator is arranged to divide each input signal into two components, and for each input signal:
to modify the current in a respective first one of the transmission lines using a first component of that input signal, and
to apply a low-pass filter to the other component of that input signal, and using the output of the low-pass filter to modify the current in the respective other one of the transmission lines;
whereby the difference between the currents in the two lines is greater for higher frequency components than for low frequency components.
Preferably, the degree to which the output of each low-pass filter modifies the respective other one of the transmission lines, is variable. This permits the transmission regulator to be tuned according to the frequency characteristics of the transmission lines.
The possibility of tuning may be achieved, for example, by selectively feeding the output of the low pass filters to a control input (e.g. gate/base) of a plurality of regulating elements (e.g. transistors) which regulate a current directed through the other one of the transmission lines. Switching elements may be provided to determine which of the regulating elements are selected. Thus, the amplification of the low frequency components may be performed using a selected combination of these regulating elements.
Varying the regulation by controlling whether or not transistors are selected has particular advantages over pre-emphasis using passive components: firstly, the manufacture of transistors can be controlled very accurately, and thus the degree of influence of the low frequency components can be controlled very accurately by controlling which transistors are used (by contrast resistors are often inaccurate); secondly, modern CMOS process targets digit technologies very aggressively, so resistors and capacitors are not always supported; thirdly, accurate transistor models for simulations are always available, but not capacitors or resistors.
Furthermore, the system according to the present invention can be arranged to ensure that the worst case pulse (e.g. a pulse which is part of a signal profile in which all the rest of the profile has opposite polarity) has a larger amplitude than would be possible by relying on a conventional passive pre-emphasis filter (i.e. there is gain).
Furthermore, the system according to the present invention can be arranged with only a low number of devices in series, so that the system is amenable to low voltage applications (say, signals of only up to 5V, or even only up to 2V).
As pointed out above, the transmission characteristics of a given transmission line are usually unknown. In this case, it is useful to provide a procedure for selecting the combination of regulating elements used, so that the characteristics of the pre-emphasis are matched to the transmission characteristics.
This can be accomplished by initially setting the switches of the regulator so that the influence of the low-pass filtered signals on the current is lowest (i.e the pre-emphasis of the high frequency portions of the signal is lowest); transmitting pulses of known length (e.g. worst casexe2x80x94that is narrowestxe2x80x94pulses); measuring the duration of the received pulse; and gradually modifying the switches to increase the influence of the low-pass filtered signals until the measured duration of the pulse is as great as the transmitted duration (to within a certain tolerance).
A second expression of the present invention is a driver for use in the system defined above.
A further aspect of the invention resides in a method of calibrating a system, including but not limited to a system as defined above, which includes a device for differentially amplifying low frequency components and high frequency components in a received signal, and in which the amplified signal is transmitted from a first end of a transmission line to a second end of the transmission line, the method including:
repeatedly generating pulses of known duration;
amplifying low frequency components and high frequency components in the generated pulses to different degrees to form a modified pulse;
transmitting the modified pulses along the line from the first end;
measuring the duration of the received pulses received at the second end; and
increasing the degree to which high frequency components are amplified relative to low frequency components until the measured duration of the received pulses is equal to the known duration of the generated pulses to within a predetermined tolerance.